As an electric power tool in which a working part is actuated by rotating a motor in normal and reverse directions, there has been known a reinforcing bar binding machine for binding reinforcing bars with a wire (for example, JP-A-2003-267307). In this reinforcing bar binding machine, a brush motor has been generally used as a drive source. The reason why this brush motor is used is because high torque can be obtained in spite of a small size, and the machine can be downsized by using the brush motor.
In the conventional reinforcing bar binding machine as described above, position sensors are disposed at several positions in the working part as means for determining timing for switching the rotation of the brush motor between normal rotation, stopping and reverse rotation so that state of the working part can be recognized depending on whether or not the motor has passed the positions of the sensors. However, a number of the sensors are required for conducting the complicated switching operation of the rotation, and in order to obtain highly accurate positional information, a step for adjusting the positions of the sensors is required.
Moreover, in case of judging a position of the motor by monitoring time on the basis of a certain reference point, there is such a problem that a large error may occur in the judged position, because a motor speed varies according to a change of battery voltage.
Further, when abrupt acceleration and deceleration are repeated in a short period, there is such anxiety that the following problems may occur.
(1) On every time of the normal rotation or the reverse rotation of the motor, an intrusive electric current flows and a winding is heated. When polarity of the brush is switched during the rotation for rapidly reversing the rotation, sparks are generated, and the brush may be worn and need to be exchanged.
(2) Until life of the motor terminates, performances of the motor (rotation number, acceleration) are deteriorated, and hence, deterioration of performances of the machine is unavoidable.
(3) The machine cannot be used in an efficient state when the performances of the motor have been deteriorated, and power consumption is increased. In case of battery driving, a battery must be charged many times, because a working amount per one charge is lowered. As the results, useful life of the battery is decreased.
Still further, an angle of a rotor and timing for energizing the winding are physically determined due to a structure of the brush motor. For this reason, in case where the motor is set so as to efficiently work in the normal rotation, the motor will not work efficiently in the reverse rotation, generating large noises. Therefore, there has been such a problem that efficient operation cannot be expected in controlling the brush motor, when the normal rotation and the reverse rotation are mixed.